Crystalline sodium aluminosilicate materials, which are able to distinguish molecules of slightly differing size and thus able to separate them from a mixture of gases are known as “molecular sieves” and quite often referred as “zeolites”. In particular, crystalline material to be named as a “zeolite,” needs to have at least one aluminum atom per unit cell, to offer catalytic activity. A number of synthetic crystalline zeolites have been prepared in the past for applications as catalysts for fluid catalytic cracking (FCC) and related operations. The most prominent and much exploited among them till date is zeolite Y. This type of zeolite with different derivatives such as NH4Y, REY, USY has been extensively used in FCC catalyst formulations.
Y zeolite is a synthetic analog of naturally occurring zeolite named faujasite, with respect to structure. The detailed synthesis of Y zeolite from sodium aluminosilicate gels with and without aging having molar composition ratios of H2O/Na2O:40, Na2O/SiO2:0.3-0.4, SiO2/Al2O3:8-20 has been discussed in Zeolite Molecular Sieves, John Wiley and Sons, D. W. Breck, 1974, pages 278 to 79. Further, the referred literature also mentions crystallization of zeolite Y associated with impurities such as A, X and P type of zeolites. Due to presence of impurities and unreasonably long crystallization time (50 to 144 hours), synthesis of Y type zeolite from above mentioned composition is not popular.
Original synthesis procedures for Y type zeolite involves prolonged aging period followed by additional period for crystallization. In general, zeolites X and Y can be synthesized from a reaction mixture comprising soda, alumina and silica in aqueous media under alkaline conditions with crystallization time ranging from 50 to 144 hours. Aging of reaction mixture can reduce crystallization time to some extent. Precursor gel once prepared and heated to crystallization temperature, zeolite growth does not start instantaneously; rather it takes some time known as “induction period” that is defined as time required for providing the conditions for “nuclei” formation. However, if analogous hydrated structure or seed is introduced to the precursor gel mixture, crystal growth occurs instantaneously over the seeds, bringing induction period to zero. Seeding not only nullify the induction period, it is an elegant technique for growing pure samples of any structure with control over crystal size at a much lower crystallization time.
Seeds of zeolite X act as ideal structures for the growth of Y as both have similar structure, the only difference being silica to alumina ratio. Nuclei of zeolite X can be easily prepared at room temperature and on transferring it to a gel mixture with appropriate composition, nuclei of X zeolite, starts growing with higher silica to alumina ratio, thus producing faujasite type zeolite. In all the prior art processes and compositions, seed preparation technique is time consuming with maturing time varying from few hours to 24 hours. Crystallization can also be initiated by the application of organic molecules having appropriate size and geometry as templates.
U.S. Pat. No. 3,306,922 refers to synthesis of Y zeolite (Na—Y) employing quaternary ammonium ions such as tetramethylammonium at 100° C. in 13 days. Such a process is economically not viable due to long crystallization period and additional cost involved due to template.
U.S. Pat. No. 3,433,589 refers to the synthesis of Y zeolite employing seed having molar composition, 1.45Na2O:1Al2O3:16.3SiO2:5.4Na2SO4:550H2O with maturing time of 24 hours at 23° C.
U.S. Pat. No. 3,789,107 refers to a seeding material having composition, in the range 5-15Na2O:3-10Na2SO4:1-4.0Al2O3:5-50SiO2:50-500H2O, for Y zeolite synthesis with aging for 24 hours at room temperature for maturity.
U.S. Pat. No. 4,016,246 refers to a process for preparing faujasite zeolite employing colloidal silica as a silica source.
U.S. Pat. No. 4,166,099 describe processes for the synthesis of faujasite type zeolite employing seed having composition, 16Na2O:1Al2O3:15SiO2:400-2000H2O with aging time between 2 to 16 hours at room temperature.
U.S. Pat. No. 4,175,059 elaborates a process for preparing a synthetic faujasite having a novel platelet-type crystalline shape with silica to alumina ratio above 2.2 by adding potassium ions to seeded faujasite synthesis slurry and heating to convert to the synthetic faujasite. The said zeolite is crystallized employing seed mixture, aged to 16 hours having molar composition, 12-20Na2O:0.75-5Al2O3:14-19SiO2:100-600H2O.
U.S. Pat. No. 4,228,137 relates to a process for an improvement in the production of zeolites, particularly of the faujasite type, employing clay based seeds derived from natural halloysite.
U.S. Pat. No. 4,340,573 relates to a seed composition of the molar ratio, 16Na2O:2-9Al2O3:15SiO2:500-2000H2O. In the said invention, the seed is aged between 15 minutes to 96 hours at temperature between 20-80° C.
U.S. Pat. No. 4,376,106 describes a process for producing Y-type zeolite with SiO2/Al2O3 molar ratio of at least 4 by employing washed gel of molar composition, 0.993Na2O:1Al2O3:3SiO2:78H2O as promoter. Preparation of this type of promoter for Y-zeolite is labor intensive and time consuming as the gel and reaction mixtures were required to be heated to 95° C. for 48 hours for obtaining Y-zeolite.
U.S. Pat. No. 4,406,822 relates to a process for simultaneous production of combined X and A zeolite promoted by powdered seed of zeolite X.
U.S. Pat. No. 4,608,236 refers to the process for production of mature seed mixture of molar composition 9-15Na2O:1Al2O3:12-18SiO2:180-300H2O that is obtained after aging for minimum of 24 hours.
U.S. Pat. No. 4,631,262 refers to a process for enhancing of storage life of a clear solution of seeds having molar ratios in the range H2O/Na2O, 15-18:Na2O/SiO2, 0-9-1.1; SiO2/Al2O3, 5.5-19; by addition of sodium silicate. The seed has been used for promoting crystallization of Y zeolite in porous microspheres of calcined clay.
U.S. Pat. No. 4,931,267 refers to the synthesis of faujasite like structure having silica to alumina ratio greater than 6, employing tetrapropyl ammonium and/or tetrabutyl ammonium and seed with the composition, 13.3Na2O:1Al2O3:12.5SiO2:267H2O.
U.S. Pat. No. 5,154,904 refers to a process for the preparation of synthetic faujasite employing seed having composition, 9-15Na2O:1Al2O3:9-15SiO2:140-260H2O; aged for the duration of 18-72 hours between 30-70° C.
U.S. Pat. No. 6,284,218 refers to a process for the preparation of high crystallite size, faujasite type zeolite by employing a seeding mixture of molar composition, 14.9Na2O:1Al2O3:14.38SiO2:319H2O where the maturation time of about 23 hours is employed.
Further, the highest crystallite size reported therein is 1700 Å. A direct correlation between crystallite size and thermal stability of NaY zeolite has been established.
The relation between thermal stability and silica to alumina ratio of zeolite framework has been extensively discussed by Julius Scherzer (“Octane-Enhancing Zeolite FCC Catalysts”, Marcel Dekker, 1990). The author has reported that thermal stability of zeolite increases with increase in silica to alumina ratio in zeolite framework. Various techniques such as thermal and hydrothermal treatment, chemical modification with EDTA and ammonium hexafluorosilicate have been described for the enhancement of silica to alumina ratio of Y zeolite framework. However, these techniques are hardware intensive and involve usage of hazardous chemicals.
From the prior art processes, it can be seen that faujasite type zeolite can be easily synthesized employing a mature seeding material having composition to that of zeolite X or zeolite Y. These compositions on subjecting to crystallization lead to the production of zeolite X or zeolite Y. However, application of mature seeds in the range 1 to 20 wt % on Y precursor gel and subjecting the mixture to a temperature of crystallization leads to the production of Y zeolite.
Zeolite A, another synthetic zeolite with framework silica to alumina ratio 2 has similar secondary building unit “sodalite” cage (14 hedron, polyhedra) in its structure as compared to that of Y zeolite. The only difference is in the sharing of face, in Y zeolite, 6-ring face is shared between sodalite cages, while in zeolite A, 4-ring face is shared between sodalite cages.
It can be inferred from the various patent and non-patent literature that the synthesis of zeolite X and Y can be carried out easily over seeds of zeolite X with silica to alumina ratio of composition centered around 3.5 to 20, that can be easily prepared at ambient temperature 25-35° C.
Till date there is no attempt to produce Y zeolite employing X-ray amorphous promoter having composition as that of zeolite A. The only exception is U.S. Pat. No. 4,007,253 that discloses the process for the production of faujasite promoted by finely divided zeolite A that is in the form of crystallized powder with particle radius less than 1 micron.
Thus, the present invention overcomes the long maturation time of the seed compositions of the prior arts and discloses a ready-to-use seed composition. The ready-to-use seed composition having zero maturation time is used in the preparation of a synthetic faujasite zeolite having high thermal stability and high crystallinity.